In the absence of an explicit `--color` setting, or when `--color=auto`
is passed, the pr diff command should fall back to respecting the global
colorization setting as inferred from the environment.
If the `--repo` flag is specified, then the user intends to select a
repository other than the current one. In that case, it doesn't make
sense to fall back to detecting the PR belonging to the current branch,
so throw a descriptive error instead.
